To solve the problem for an object with thickness, imagine shifting the wall on the inside of the turn in by a distance equal to the width of a box (with the corner expanding out to a semicircle), and then consider moving a line segment through the appropriately shrunken hallway.
Note that we're still being possibly inefficient, as we are not using our third available dimension; however, this is only relevant if the box is much longer in one direction than the other two (for example, if you were trying to carry a 2x4 around this corner).